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In 2008, Dr. Khoruts, a gastroenterologist at the University of Minnesota, took on a patient suffering from a vicious gut infection of Clostridium difficile. She was crippled by constant diarrhea, which had left her in a wheelchair wearing diapers. Dr. Khoruts treated her with an assortment of antibiotics, but nothing could stop the bacteria. His patient was wasting away, losing 60 pounds over the course of eight months. “She was just dwindling down the drain, and she probably would have died,” Dr. Khoruts said.

Dr. Khoruts decided his patient needed a transplant. But he didn’t give her a piece of someone else’s intestines, or a stomach, or any other organ. Instead, he gave her some of her husband’s bacteria.

Dr. Khoruts mixed a small sample of her husband’s stool with saline solution and delivered it into her colon. Writing in the Journal of Clinical Gastroenterology last month, Dr. Khoruts and his colleagues reported that her diarrhea vanished in a day. Her Clostridium difficile infection disappeared as well and has not returned since.

The procedure — known as bacteriotherapy or fecal transplantation — had been carried out a few times over the past few decades. But Dr. Khoruts and his colleagues were able to do something previous doctors could not: they took a genetic survey of the bacteria in her intestines before and after the transplant.

Before the transplant, they found, her gut flora was in a desperate state. “The normal bacteria just didn’t exist in her,” said Dr. Khoruts. “She was colonized by all sorts of misfits.”

Two weeks after the transplant, the scientists analyzed the microbes again. Her husband’s microbes had taken over. “That community was able to function and cure her disease in a matter of days,” said Janet Jansson, a microbial ecologist at Lawrence Berkeley National Laboratory and a co-author of the paper. “I didn’t expect it to work. The project blew me away.”

Scientists are regularly blown away by the complexity, power, and sheer number of microbes that live in our bodies. “We have over 10 times more microbes than human cells in our bodies,” said George Weinstock of Washington University in St. Louis. But the microbiome, as it’s known, remains mostly a mystery. “It’s as if we have these other organs, and yet these are parts of our bodies we know nothing about.”

Dr. Weinstock is part of an international effort to shed light on those puzzling organs. He and his colleagues are cataloging thousands of new microbe species by gathering their DNA sequences. Meanwhile, other scientists are running experiments to figure out what those microbes are actually doing. They’re finding that the microbiome does a lot to keep us in good health. Ultimately, researchers hope, they will learn enough about the microbiome to enlist it in the fight against diseases.

“In just the last year, it really went from a small cottage industry to the big time,” said David Relman of Stanford University.

The microbiome first came to light in the mid-1600s, when the Dutch lens-grinder Antonie van Leeuwenhoek scraped the scum off his teeth, placed it under a microscope and discovered that it contained swimming creatures. Later generations of microbiologists continued to study microbes from our bodies, but they could only study the ones that could survive in a laboratory. For many species, this exile meant death.

In recent years, scientists have started to survey the microbiome in a new way: by gathering DNA. They scrape the skin or take a cheek swab and pull out the genetic material. Getting the DNA is fairly easy. Sequencing and making sense of it is hard, however, because a single sample may yield millions of fragments of DNA from hundreds of different species.

A number of teams are working together to tackle this problem in a systematic way. Dr. Weinstock is part of the biggest of these initiatives, known as the Human Microbiome Project. The $150 million initiative was started in 2007 by the National Institutes of Health. The project team is gathering samples from 18 different sites on the bodies of 300 volunteers.

To make sense of the genes that they’re gathering, they are sequencing the entire genomes of some 900 species that have been cultivated in the lab. Before the project, scientists had only sequenced about 20 species in the microbiome. In May, the scientists published details on the first 178 genomes. They discovered 29,693 genes that are unlike any known genes. (The entire human genome contains only around 20,000 protein-coding genes.)

“This was quite surprising to us, because these are organisms that have been studied for a long time,” said Karen E. Nelson of the J. Craig Venter Institute in Rockville, Md.

The new surveys are helping scientists understand the many ecosystems our bodies offer microbes. In the mouth alone, Dr. Relman estimates, there are between 500 and 1,000 species. “It hasn’t reached a plateau yet: the more people you look at, the more species you get,” he said. The mouth in turn is divided up into smaller ecosystems, like the tongue, the gums, the teeth. Each tooth—and even each side of each tooth—has a different combination of species.

Scientists are even discovering ecosystems in our bodies where they weren’t supposed to exist. Lungs have traditionally been considered to be sterile because microbiologists have never been able to rear microbes from them. A team of scientists at Imperial College London recently went hunting for DNA instead. Analyzing lung samples from healthy volunteers, they discovered 128 species of bacteria. Every square centimeter of our lungs is home to 2,000 microbes.

Some microbes can only survive in one part of the body, while others are more cosmopolitan. And the species found in one person’s body may be missing from another’s. Out of the 500 to 1,000 species of microbes identified in people’s mouths, for example, only about 100 to 200 live in any one person’s mouth at any given moment. Only 13 percent of the species on two people’s hands are the same. Only 17 percent of the species living on one person’s left hand also live on the right one.

This variation means that the total number of genes in the human microbiome must be colossal. European and Chinese researchers recently catalogued all the microbial genes in stool samples they collected from 124 individuals. In March, they published a list of 3.3 million genes.

The variation in our microbiomes emerges the moment we are born.

“You have a sterile baby coming from a germ-free environment into the world,” said Maria Dominguez-Bello, a microbiologist at the University of Puerto Rico. Recently, she and her colleagues studied how sterile babies get colonized in a hospital in the Venezuelan city of Puerto Ayacucho. They took samples from the bodies of newborns within minutes of birth. They found that babies born vaginally were coated with microbes from their mothers’ birth canals. But babies born by Caesarean section were covered in microbes typically found on the skin of adults.

“Our bet was that the Caesarean section babies were sterile, but it’s like they’re magnets,” said Dr. Dominguez-Bello.

We continue to be colonized every day of our lives. “Surrounding us and infusing us is this cloud of microbes,” said Jeffrey Gordon of Washington University. We end up with different species, but those species generally carry out the same essential chemistry that we need to survive. One of those tasks is breaking down complex plant molecules. “We have a pathetic number of enzymes encoded in the human genome, whereas microbes have a large arsenal,” said Dr. Gordon.

In addition to helping us digest, the microbiome helps us in many other ways. The microbes in our nose, for example, make antibiotics that can kill the dangerous pathogens we sniff. Our bodies wait for signals from microbes in order to fully develop. When scientists rear mice without any germ in their bodies, the mice end up with stunted intestines.

In order to co-exist with our microbiome, our immune system has to be able to tolerate thousands of harmless species, while attacking pathogens. Scientists are finding that the microbiome itself guides the immune system to the proper balance.

One way the immune system fights pathogens is with inflammation. Too much inflammation can be harmful, so we have immune cells that produce inflammation-reducing signals. Last month, Sarkis Mazmanian and June L. Round at Caltech reported that mice reared without a microbiome can’t produce an inflammation-reducing molecule called IL-10.

The scientists then inoculated the mice with a single species of gut bacteria, known as Bacteroides fragilis. Once the bacteria began to breed in the guts of the mice, they produced a signal that was taken up by certain immune cells. In response to the signal, the cells developed the ability to produce IL-10.

Scientists are not just finding new links between the microbiome and our health. They’re also finding that many diseases are accompanied by dramatic changes in the makeup of our inner ecosystems. The Imperial College team that discovered microbes in the lungs, for example, also discovered that people with asthma have a different collection of microbes than healthy people. Obese people also have a different set of species in their guts than people of normal weight.

In some cases, new microbes may simply move into our bodies when disease alters the landscape. In other cases, however, the microbes may help give rise to the disease. Some surveys suggest that babies delivered by Caesarian section are more likely to get skin infections from multiply-resistant Staphylococcus aureus. It’s possible that they lack the defensive shield of microbes from their mother’s birth canal.

Caesarean sections have also been linked to an increase in asthma and allergies in children. So have the increased use of antibiotics in the United States and other developed countries. Children who live on farms — where they can get a healthy dose of microbes from the soil — are less prone to getting autoimmune disorders than children who grow up in cities.

Some scientists argue that these studies all point to the same conclusion: when children are deprived of their normal supply of microbes, their immune systems get a poor education. In some people, untutored immune cells become too eager to unleash a storm of inflammation. Instead of killing off invaders, they only damage the host’s own body.

A better understanding of the microbiome might give doctors a new way to fight some of these diseases. For more than a century, scientists have been investigating how to treat patients with beneficial bacteria. But probiotics, as they’re sometimes called, have only had limited success. The problem may lie in our ignorance of precisely how most microbes in our bodies affect our health.

Dr. Khoruts and his colleagues have carried out 15 more fecal transplants, 13 of which cured their patients. They’re now analyzing the microbiome of their patients to figure out precisely which species are wiping out the Clostridium difficile infections. Instead of a crude transplant, Dr. Khoruts hopes that eventually he can give his patients what he jokingly calls “God’s probiotic” — a pill containing microbes whose ability to fight infections has been scientifically validated.

Dr. Weinstock, however, warns that a deep understanding of the microbiome is a long way off.

“In terms of hard-boiled science, we’re falling short of the mark,” he said. A better picture of the microbiome will only emerge once scientists can use the genetic information Dr. Weinstock and his colleagues are gathering to run many more experiments.

“It’s just old-time science. There are no short-cuts around that,” he said.

Eileen Power DePaula, M.Ed., M.Ac., Lic.Ac., has been practicing acupuncture since 1995. She has degrees in Clinical Nutrition, Massage Therapy, Health Education, and most recently completed an Integrative Health Coach Training. She is passionate about food, nutrition, and maintaining optimal health and quality of life.